One conventional circuit board includes pads which are formed using nickel and gold immersion technologies. These circuit boards typically include standard copper traces supported by layers of fiberglass (e.g., FR4 fiberglass) which are glued together. Some of the copper traces terminate at nickel/gold surface mount pads, i.e., nickel pads plated with a gold finish, which are configured to solder to surface mount devices such as Ball Grid Array (BGA) devices.
To manufacture a circuit board having nickel/gold immersion pads, a manufacturer typically forms individual circuit board layers, i.e., individual layers of fiberglass having etched copper traces thereon, using an acid etch process. Next, the manufacturer typically glues the individual layers together, as well as cuts and drills edges, grooves and holes to form the section of circuit board material. Then, the manufacturer typically forms nickel pads and nickel plated-through holes by exposing the section of circuit board material to a nickel immersion process, e.g., by placing the circuit board section in a bath containing nickel and other additives that facilitate reliable and consistent pad growth such as phosphates, sulfides, etc. Next, the manufacturer typically provides a protective coating of gold to inhibit nickel oxidation. It should be understood that the manufacturer typically performs additional cleaning steps (e.g., rinsing steps) along the way (e.g., interspersed within the above-described series of manufacturing steps) to clean the section of circuit board material of residues, byproducts and contaminants.
The end result circuit board includes a section of circuit board material which supports a set of virgin surface mount nickel/gold pads (surface mount pads which previously have not been soldered to a circuit board component but which are ready for soldering to a circuit board component) and a set of virgin nickel/gold plated-through holes. A typical virgin surface mount nickel/gold pad includes a layer of nickel which is approximately 200 to 250 micro-inches in thickness, and a top layer of gold which is approximately 6 micro-inches in thickness (e.g., 3 to 8 micro-inches). When circuit board components mount to the virgin pads, molten solder forms solderjoints between the pads and the leads (i.e., contacts) of the circuit board components. During the soldering process, the protective gold finish typically melts away (e.g., mixes with the solder) leaving solder to form intermetallic boundaries between the surface mount pads of the circuit board and the component leads.
Populated circuit boards, which include nickel/gold surface mount pads, occasionally suffer from xe2x80x9cBlack Padxe2x80x9d defects. A xe2x80x9cBlack Padxe2x80x9d defect is a flaw (e.g., a fracture) in an intermetallic boundary between a nickel surface mount pad and a lead of a circuit board component that results in an electrically unreliable connection between that nickel pad and that component lead. Such a defect often appears as a pressure-sensitive intermittent electrical connection between the device and the circuit board, i.e., between the nickel pad and the component lead. Studies have shown that xe2x80x9cBlack Padxe2x80x9d defects can be caused by excessive pad corrosion (i.e., oxidation of the nickel layer) prior to soldering. Such corrosion results in low solder-wettability (i.e., a low affinity for solder) thus providing a weak and unreliable solder joint after the soldering process.
Circuit board manufacturers can take a variety of approaches to handling xe2x80x9cBlack Padxe2x80x9d defects. One conventional approach involves the manufacturer inspecting each populated circuit board for xe2x80x9cBlack Padxe2x80x9d defects, and simply throwing away any circuit board having a xe2x80x9cBlack Padxe2x80x9d defect. Another conventional approach involves the manufacturer inspecting each populated circuit board for xe2x80x9cBlack Padxe2x80x9d defects, and reworking any circuit board having a xe2x80x9cBlack Padxe2x80x9d defect, i.e., unsoldering a circuit board component exhibiting symptoms of having a solder joint to a xe2x80x9cBlack Padxe2x80x9d, cleaning the exposed surface mount pads, and soldering on a new circuit board component. Yet another conventional approach involves the fabrication manufacturer redesigning the board fabrication process to avoid using surface mount pads formed by nickel/gold immersion (e.g., redesigning the circuit board manufacturing process to use bare copper pads, silver pads, palladium pads, etc.).
Unfortunately, there are deficiencies to the above-described conventional approaches to avoiding xe2x80x9cBlack Padxe2x80x9d defects. For example, in the above-described conventional approach which involves throwing away populated circuit boards, a significant amount of added value is lost. In particular, some circuit boards may cost several thousands of dollars to make and it may be a significant drawback for a company to bear the burden of regularly writing-off such a cost.
Additionally, in the above-described conventional approach which involves reworking a populated circuit board having a xe2x80x9cBlack Padxe2x80x9d defect, the rework process does not consistently and effectively repair the intermittent connection caused by the xe2x80x9cBlack Padxe2x80x9d defect. That is, the intermittent connection is often formed by flaws in the intermetallic boundaries of the nickel layers of the metallic pads and, as such, is not fixed by simply replacing a circuit board component. To the contrary, a metallic pad suffering from a xe2x80x9cBlack Padxe2x80x9d defect typically has corrosion which extends below the pad surface (e.g., 20 micro-inches below the pad surface) as well as low solder-wetting ability (i.e., low affinity for solder) which does not improve when a new component lead is soldered to the pad. Accordingly, any new solder joint formed on the metallic pad is also likely to be unreliable and prone to failure.
Furthermore, in the above-described conventional approach which involves modifying the circuit board manufacturing process to use other types of pads (e.g., bare copper pads, silver pads, palladium pads), the alternative circuit board manufacturing processes can be more susceptible to other deficiencies which are not present in circuit boards using a nickel/gold immersion processes. For example, circuit boards, which use nickel/gold immersion where the nickel overplates the via copper and forms nickel eyelets, are well-suited to slowing down mechanical expansion of the circuit board in the Z-direction (i.e., circuit board expansion which is perpendicular to the circuit board plane) due to the clamping force provided by the nickel thus avoiding other circuit board drawbacks such as warping, fractures in metallic traces, separation of circuit board layers, etc. Eyelets formed of other metals have not inhibited circuit board expansion in the Z-direction as well as nickel eyelets. Accordingly, eliminating the nickel/gold eyelets and using other metallic eyelets (e.g., copper, silver, palladium, etc.) can provide poorer circuit board expansion results and thus promote other circuit board drawbacks.
The invention is directed toward techniques for manufacturing a circuit board having virgin metallic surface mount pads which involve removing a portion of each virgin metallic surface mount pad (e.g., removing several micro-inches from the tops of pads formed by a nickel immersion process). Accordingly, any corrosion or contaminants which collected within these removed portions are no longer available to promote xe2x80x9cBlack Padxe2x80x9d defects.
For example, phosphate compounds, which typically reside within nickel immersion baths to control nickel deposition rates, can become incorporated into the nickel immersion pads. In particular, such phosphate compounds can collect near the top surfaces of nickel immersion pads. Although these phosphate compounds in theory are supposed to provide metallic properties, these phosphate compounds may actually operate more like organic contaminants that interfere with formation of healthy solder joints (i.e., may lower solder-wettability of the pads). Removal of these contaminated top surfaces prior to the soldering process promotes formation of robust and healthy solder joints for thorough electrical and structural connectivity.
One embodiment of the invention is directed to a circuit board manufacturing system having a paste source, a circuit board processing apparatus coupled to the paste source, and a controller coupled to the circuit board processing apparatus. The circuit board processing apparatus includes a carrier which is configured to receive a circuit board having (i) a section of circuit board material and (ii) virgin metallic surface mount pads (e.g., pads formed by a nickel immersion process) which are supported by the section of circuit board material. The circuit board processing apparatus further includes a paste distribution assembly (e.g., a manifold and set of nozzles) coupled to the carrier and to the paste source. The paste distribution assembly is configured to dispose a paste (e.g., a compound including carbohydroxilate flux and an abrasive material) from the paste source onto a surface of the circuit board. The carrier further includes a surfacing assembly coupled to the carrier. The surfacing assembly is configured to move the paste over the surface of the circuit board to remove a portion of each virgin metallic surface mount pad. The controller (e.g., an electronic control device, a computer, etc.) is configured to selectively start and stop operations of the paste distribution and surfacing assemblies.
It should be understood that movement of the paste over the metallic pads enables removal of a portion of each pad in a chemical and physical manner. That is, flux within the paste reacts with the metal in each pad. At the same time, movement of the paste (e.g., movement of abrasive material within the paste over the pads) exposes additional surfaces of the pads for reaction with the flux (e.g., a flux loosely based on a Carbon flux chemistry).
In one arrangement, the paste includes non-flammable carbohydroxilate flux and abrasive material. In this arrangement, the surfacing assembly includes a roller having fiber-reinforced polyester material, and a positioning member which is configured to position and operate the roller over the paste and the surface of the circuit board. In one arrangement, the roller removes the portions of the pads in a lapping manner (e.g., gradually by polishing material off of the top surface of the pads). In one arrangement, the non-flammable carbohydroxilate flux provides both lubrication and heat control between the roller and the surface of the circuit board. The above-described lapping operation is essentially self-limiting since the flux reacts only with the exposed surfaces of the pads (i.e., the amount of each pad which is removed is greater than the oxide thickness or contamination thickness, but not so much that the pad becomes no longer useful for mounting to circuit board component contacts).
In one arrangement, the circuit board processing apparatus further includes a heater which is configured to heat the paste to a temperature of at least 150 degrees Fahrenheit (e.g., substantially to 160 degrees Fahrenheit). This application of heat facilitates chemical reaction between the flux within the paste and the virgin metallic surface mount pads.
In one arrangement, the positioning member applies the roller to the paste and the surface of the circuit board in an even manner such that, after the portion of each virgin metallic surface mount pad is removed, the virgin metallic surface mount pads have substantially the same height. Accordingly, circuit board components (e.g., BGA devices) can sit over the pads in a uniform and stable manner thus avoiding gaps or unnecessarily long solder joints between particular pads and component leads.
In some arrangements, the circuit board manufacturing system processes both sides of the circuit board simultaneously. For example, in one arrangement, the paste distribution assembly includes a first paste dispenser (e.g., a first set of nozzles) which is configured to dispense the paste on a first side of the circuit board, and a second paste dispenser (e.g., a second set of nozzles) which is configured to dispense paste on a second side of the circuit board which is opposite the first side. In this arrangement, the surfacing assembly includes a first set of rollers which is configured to move the paste and the first side of the circuit board, and a second set of rollers which is configured to move the paste and the second side of the circuit board. The first and second sets of rollers can be configured to apply even pressure for uniform processing of both sides of the circuit board at substantially the same time.